Power devices operated in integrated circuits typically operate with a voltage in the range 20V to 1.2 kV and typically higher than 30V or 50V or so. Power devices typically operate with a current in the range 10 mA to 50A and typically higher than 0.1A and smaller than 5 A. Such devices may also be referred to as “high voltage/power devices”. These devices are typically capable of delivering from a few mWatts to 1 Watt or even a few tens of Watts of power. Their application may range from domestic appliances, electric cars, motor control, and power supplies to RF and microwave circuits and telecommunication systems.
It will be appreciated that the terms “top” and “bottom”, “above” and “below”, and “lateral” and “vertical”, may be used in this specification by convention and that no particular physical orientation of the device as a whole is implied.
Lateral devices in integrated circuits have the main terminals (variously called the anode/cathode, drain/source and emitter/collector) and the control terminals (termed the gate or base) placed at the top surface of the device in order to be easily accessible. In power ICs, such devices are often monolithically integrated with CMOS-type or BiCMOS-type low voltage/low power circuits. It is desirable that several high voltage/power devices are integrated within the same chip.
MOS bipolar power devices, such as the lateral insulated gate bipolar transistor (LIGBT), are based on MOS control with bipolar current conduction in the lowly-doped drift layer or region of the device. Such devices are based on the conductivity modulation concept. At high levels of charge injection, when the current in the device increases, a mobile charge of electrons and of holes is built up in the drift layer, leading to a desirably sharp increase in the conductivity of the drift layer. The mobile charge accumulated when the device is in the on-state dictates the on-state/switching performance of the device given that the mobile carriers must be removed or “mopped up” in order to switch the device to the off-state.
MOS-bipolar devices, such as the LIGBT, can be broadly regarded as a low voltage MOS component driving a wide base (high voltage) bipolar transistor. By way of example, an n-channel LIGBT has an n-channel MOSFET driving the base of a pnp transistor. Such devices do not normally have reverse current conduction because, unlike in a MOSFET (or power MOSFET), there is no associated integral body diode. As is known, the integral body diode in a MOSFET is an intrinsic component of the MOS transistor and is connected in an anti-parallel configuration and allows for reverse current conduction.
In some applications and some architectures, such as half bridges or full bridges, reverse current conduction is necessary. In this respect, therefore, the MOSFET has an advantage over the IGBT in such applications as it has an intrinsic anti-parallel body diode.
Anode-shorted IGBTs are based on a combination of a MOSFET with an IGBT. They behave as MOSFETs until a certain current threshold is reached, above which they operate as an IGBT. These devices are generally faster than the conventional IGBTs and feature a body diode. A significant drawback of such arrangements however is that a sharp snap-back is seen in the output characteristics when the device commutes from the MOSFET mode (unipolar mode) to the IGBT mode (bipolar mode). There is therefore a very difficult trade-off in anode-shorted devices between the IGBT and the body diode performance. In general, a high performance diode comes with a significant snap-back and thus is an unacceptable solution. Reducing the snap-back however kills the diode output power to an extent that is almost un-usable.
Examples of anode-shorted IGBTs or double gate devices are disclosed in F. Udrea, G. A. J. Amaratunga, J. Humphrey, J. Clark and A. Evans, “The MOS Inversion Layer as a Minority Carrier Injector”, IEEE, Electron Device Letters, volume 17, no. 9, p. 425, September 1996; U. N. K. Udugampola, R. A. McMahon, F. Udrea, K. Sheng, G. A. J. Amaratunga, E. M. S. Narayanan, S. Hardikar, and M. M. De Souza, “Dual Gate Lateral Inversion Layer Emitter Transistor for Power and High Voltage Integrated Circuits”, International Symposium on power semiconductor devices and ICs, Cambridge 2003, p. 216-219; F. Udrea, U. N. K. Udugampola, K. Sheng, R. A. McMahon, G. A. J. Amaratunga, E. M. S. Narayanan, M. M. De Souza, and S. Hardikar, “Experimental Demonstration of an Ultra-fast Double Gate Inversion Layer Emitter Transistor (DG-ILET)”, IEEE Electron Device Letters, Volume 23, Issue 12, December 2002, p. 725-727; and H. Takahashi et al, “1200 V Reverse Conducting IGBT”, ISPSD, p. 133, 2004.
Using a smart design in vertical devices, it is possible to build a relatively good trade-off between the diode and the IGBT performance, but still the conduction paths for the diodes and the IGBT are not exactly the same, which in effect means that an increase in area of the device is necessary to accommodate both components in one die.